1. Field of the Invention
The present invention relates to an optical switch system provided with a plurality of input ports and a plurality of output ports, and more specifically to an optical packet switch system for switching an optical packet input through an arbitrary input port to an arbitrary output port.
2. Description of the Related Art
Conventionally, metal wiring has been used for a connection between devices (for example, for communications between computers), a connection between boards (for example, for communications between printed circuit boards), and a connection between elements in a board (for example, for communications between elements in a printed circuit board). However, in metal wiring, there is a problem of a transmission loss or restrictions of a transmission band. The speed of the evolution in an LSI chip including a CPU greatly exceeds the speed of the evolution of the high-speed technology of electric wiring. Therefore, in a system using electric wiring, the processing speed of an LSI cannot efficiently work. Additionally, with the remarkable improvement of the capacity of LSI chips, the number of input/output pins provided for an LSI has reached several hundreds or thousands. Therefore, it has been difficult to connect an LSI having such a large number of input/output pins with metal wiring.
Recently, optical interconnect technology has received attention as a breakthrough for solving the problem (wiring bottleneck) about the metal wiring. An optical interconnect generally refers to the optical data communications for a very short distance, and commonly indicates optical communication for a distance shorter than a communication distance in a LAN system.
The optical interconnect can be used in connecting devices, boards, and elements in a board. That is, the optical interconnect can be used in transmitting a signal between, for example, the above-mentioned LSIs. Otherwise, in a parallel computer system connecting a plurality of computers or a signal switching circuit of a high-speed router device, the optical interconnect may be used to solve the bottleneck (bandwidth, power consumption, generated heat, size of cable, etc.) of electric wiring technology.
FIG. 1 shows an example of an optical packet switch system using the optical interconnect technology. In this example, the configuration with distribution/selection using semiconductor optical amplifiers (SOA) is shown in FIG. 1. The switch configuration shown in FIG. 1 is a 4×4 switch.
Each input port (#1 through #4) is provided with an optical coupler 1 for distributing an input optical packet signal to output ports (#1 through #4). Each output port (#1 through #4) is provided with semiconductor optical amplifiers 2-1 through 2-4, an optical wavelength multiplexing coupler 3, and a semiconductor optical amplifier (SOA) 4. Each of the semiconductor optical amplifiers 2-1 through 2-4 operates as a gate switch, and passes or rejects an optical packet signal. The optical wavelength multiplexing coupler 3 multiplexes optical packet signals output from the semiconductor optical amplifiers 2-1 through 2-4. The semiconductor optical amplifier 4 amplifies the optical packet signal output from the optical wavelength multiplexing coupler 3 and outputs it. With the above-mentioned two-stage amplification configuration, the reduction of the crosstalk element from an adjacent port and the improvement of an optical SN ratio are realized.
In FIG. 1, the optical packet #1-1 input from the input port (#1), the optical packet #2-1 input from the input port (#2), the optical packet #3-1 input from the input port (#3), and the optical packet #4-1 input from the input port (#4) are sequentially output to the output port (#1).
In the optical packet switch system with the above-mentioned configuration, there can be time difference in optical packet arrival between the input ports. However, in the present technology, the optical buffer element (or a delay element) for holding an optical packet as an optical signal and compensating for the arrival time difference has not practically realized. Therefore, in the existing optical packet switch system, normally the transmission timing of an optical packet is synchronized, and timing of the ON/OFF operation of the semiconductor optical amplifiers 2-1 through 2-4 is adjusted in each output port, thereby realizing switching.
The technique of controlling the output level of a semiconductor optical amplifier is, for example, described in the document 1. In the control method described in the document, the output power of an optical signal light can be maintained at a predetermined level by appropriately adjusting the power of control light applied to a semiconductor optical amplifier.
[Document 1] Ken Morito, “Output-Level Control of Semiconductor Optical Amplifier by External Light Injection”, Journal Of Lightwave Technology, Vol. 23, No. 12, December 2005
However, the output power of each transmitter (or light source) for transmitting an optical packet is not the same as that of another transmitter. The loss in the transmission line, and the loss in the connector are not the same as those of each optical path. Therefore, when optical packet signals passing optical gate switches (that is, the semiconductor optical amplifiers 2-1 through 2-4) are multiplexed, by the optical wavelength multiplexing coupler 3, the power level of each optical packet signal can be varied among the input ports. In the example shown in FIG. 2, the power of the optical packet signals #1-1 and #4-1 is large and the power of the optical packet signal #3-1 is small in the output port (#1). In this case, the receiver for receiving the optical packet string (#1-1, #2-1, #3-1, and #4-1) requires a large dynamic range, and it is necessary to provide an expensive or complicated circuit.